Concept: Electrical conduction needs free or delocalised electrons. In diamond, there are no free electrons because all valence electrons are trapped.Explanation: Diamond is a giant covalent structure. Each carbon atom uses all four valence electrons to form strong covalent bonds with neighbouring carbon atoms. These bonding electrons are localised between the atoms. So no mobile electrons exist to carry an electric current. Therefore diamond acts as an insulator. The bonding electrons are not involved in metallic, coordinate, or crystal-lattice formations as free carriers. They are entirely used in covalent bond formation.Answer: B. Covalent bond formation.